Michelle Kwan in Space

Michelle Kwan in Space

by Adam Frank for McGraw-Hill

Everywhere you look in the heavens it seems certain shapes pop up again and again. With all the round stars and planets out there Nature clearly has a thing for spheres. But sphere aren't the only familiar shapes nature seems to build with in the heavens. Even a short review of an introductory astronomy textbook will show you that disks pop up in a lot of places too. Many galaxies have a disk shape (our own Milky Way for example). The orbits of the planets in our solar system form a disk. There is pretty good evidence for disks of gas circling around monster black holes at the center of distant galaxies and snapshots of newly formed stars clearly show the young sun's embedded in disks or gas and dust. Nature's penchant for spheres is pretty is easy to understand if you consider that gravity is always pulling inward from every direction towards the center of a massive object. The origin of disks however doesn't pop out at you so easily though.

Understanding why Nature constructs so many disk-shaped structures in the cosmos is not too hard to wrap your brain around if you have two basic elements. The first thing you will need in your mental toolkit is gravity which, as we all know, sucks. Pretty much every recognizable structure in space (galaxies, stars etc) began as a giant cloud of cosmic gas. Now if you take a gas cloud floating in space and make it massive enough and pack it together tightly enough, sooner or later it will collapse under its own gravitational force. Gravity makes the gas fall inward and, if left to its own devices, will create something in the shape of a sphere as material rains down towards the center.

The next element you need to think about is rotation. Rotating stuff behaves in very special and very different ways from static or non-rotating stuff. (I was about to use the word spinning but I stopped myself because you have to be careful. Spin is a particular form of rotation. A planet can rotate about the sun with out spinning about its own axis.) One of the most important things about rotating masses is they obey the law of Conservation of Angular Momentum. That is big way of saying there is a relationship between an objects rotation speed and its size. This is something we have all seen in the Olympics. When a spinning figure skater pulls her arms inwards she spins faster. If she extends her arms she spins slower. On a certain level that is all there to the conservation of angular Momentum.

What does all this have to do with astronomy? Remember the giant gas clouds I mentioned before, the ones we all began from? Those clouds aren't static. They are turbulent messes of roiling and tumbling motions and that means rotation. So, if you take a big, slowly rotating chunk of cosmic gas cloud and let it collapse, conservation of angular momentum says you must end up a smaller, rapidly rotating chunk of a cosmic gas cloud. The cosmic significance of all this rapid rotation is as easy to understand as the time you squished (or got squished) by your brother in the back seat of the family car as you rounded a turn. The centrifugal "force" of the turn pushed you outwards away from the direction of the turn. In the exact same way the rapid rotation of gas in the collapsing cloud can slow its inward motion. Of course not all the gas is rotating. If you start with a spinning ball of gas, the material long the poles isn't spinning at all. Once the cloud stars collapse it won't spin-up and won't feel any outward centripetal push.

So now you can see why Nature keeps creating disks. If a rotating cloud begins to collapse, the parts of the cloud near the axis will fall towards the center unimpeded. The parts of the cloud at what we could call the equator where the rotation is fastest will rotate faster and faster the closer they get to center. Eventually they can be spinning so around the center so fast that gravity and the centrifugal force balance. Viola! What started as a sphere ends up looking like a Frisbee.

There are some important details, like the role of friction, which we are missing but I hope you get the picture. There is a profound and subtle method in the midst of Nature's seeming chaos. How else could you take a messy thing like a cloud and build something as structured and ordered as a solar system.

Questions to Ponder

What other examples of conservation of angular momentum can you think of in real life?
What other examples of disks or disk-like structures can you think of in astronomy?
What other kinds of shapes occur in Nature (on Earth and in space?)

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